The present invention relates generally to semiconductor diodes. More specifically, the present invention relates to a double barrier tunnel diode.
A diode is a semiconductor device having a non-linear voltage-current relationship. Diodes are important solid-state devices and are used in many electronic applications. The tunnel diode is one of a variety of diodes having the characteristic of negative resistance. As the term is used, negative resistance is a voltage-current behavior wherein, over certain voltage ranges, increasing the voltage applied across the diode leads to decreased current carried through the diode. By contrast, in most devices, an increasing applied voltage results in increasing current. Tunnel diodes have a number of applications, including high frequency oscillator circuits and high frequency electronic circuits.
One type of tunnel diode is the double barrier tunnel diode. One known double barrier tunnel diode includes a gallium arsenide quantum well with a thin barrier layer of aluminum gallium arsenide epitaxially joined to each side of the quantum well. This structure, termed a quantum barrier, in turn lies between two injection layers of gallium arsenide. The quantum barrier creates an energy barrier to the flow of electrons that can be overcome by electrons only under certain conditions, and which results in the negative resistance characteristic of interest over a range of applied bias voltage. Electrons are injected into the quantum barrier from the conduction band of one of the injection layers under an internal voltage produced by the applied external voltage. The internal voltage increases the energy of the injected electrons to satisfy the resonant tunneling condition of the quantum barrier. Under the proper conditions of voltage sufficient to satisfy the resonant condition, the incoming electron has the same energy as the resonance level in the quantum well. These electrons tunnel through the quantum barrier. As the bias voltage is increased further, the energy levels no longer align and the current decreases, resulting in the negative resistance effect.
Another known double barrier tunnel diode is disclosed in U.S. Pat. No. 5,296,721. In this diode, the valley current is decreased in a resonant tunneling diode by introducing strain into the well region. The barrier layers have a biaxially strained epitaxial relationship with the material of the quantum well. The biaxial strain is sufficient to reduce the energy of the heavy holes in the quantum well to less than the energy of the conduction band minimum energy of the electron injection layer.
One problem with prior known devices is that they are not easily tailored for specific high-speed circuit applications, such as high-speed signal processing. These devices include comparators, digital to analog converters, sample and hold circuits, logic, and frequency multipliers. In many circuit applications, the negative resistance portion of the current voltage curve is not optimized. Thus, power consumption and heat production, as well as noise sensitivity, is increased.
It is, therefore, one object of the invention to provide a resonant tunneling diode construction that is capable of being easily modified and manufactured to reduce power consumption and heat generation in circuits and decrease sensitivity to noise. It is a further object of the invention to provide a resonant tunneling diode capable of less power consumption, particularly at microwave frequencies of tens of gigahertz in bandwidth.
In one aspect of the invention, a double barrier tunnel diode has a quantum well with at least one layer of semiconductor material. The tunnel diode also has a pair of injection layers on either side of the quantum well. The injection layers comprise a collector layer and an emitter layer. A barrier layer is positioned between each of the injection layers and the quantum well. The quantum well has an epitaxial relationship with the emitter layer. The amount of one element of the well layer is increased to increase the lattice constant a predetermined amount. This may cause a reduction in the resonant energy level. A second element is added to the well layer to increase the resonant energy level, but not to change the lattice constant. By controlling the composition in this matter, the effective mass, and thus the negative resistance, may be controlled for various diode constructions while retaining some freedom to adjust the resonant energy level in the quantum well.
One advantage of the invention is that by varying the effective mass in the well, the width of the voltage range of the negative resistance region of the resonant tunneling diode may easily be controlled during manufacture.
In the preferred structure, the well composition is modified from a commonly known structure, which uses just InGaAs. In the present invention, InGaAlAs is used in the well. By substituting aluminum for gallium in the well, only a small effect on the lattice constant is achieved. However, the resonant energy of the conduction band increases. In the present invention, the conduction band level and the lattice constant may be adjusted independently.
Other features and advantages of the present invention will be apparent from the following detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.